The present invention relates to fiber optic gyroscopes used for rotation sensing and, more particularly, to fiber optic gyroscopes operated in a feedback loop.
Fiber optic gyroscopes are an attractive means with which to sense rotation. They can be made quite small and still be constructed to withstand considerable mechanical shock, temperature change, and other environmental extremes. In the absence of moving parts, they can be nearly maintenance free, and they have the potential of becoming economical in cost. They can also be sensitive to low rotation rates that can be a problem in other kinds of optical gyroscopes.
A fiber optic gyroscope has a coil of optical fiber wound on a core and about the axis thereof around which rotation is to be sensed. The optical fiber is typically of a length of 1,000 to 2,000 meters, or so, and is part of a closed optical path in which an electromagnetic wave, or light wave, is introduced and split into a pair of such waves, to propagate in opposite directions through the coil to both ultimately impinge on a photodetector. Rotation about the sensing axis of the core, or of the coiled optical fiber, provides an effective optical path length increase in one rotational direction and an optical path length decrease in the other rotational direction for one of these waves. The opposite result occurs for rotation in the other directions. Such path length differences between the waves introduce a phase shift between these waves for either rotation direction, i.e. the well known Sagnac effect. The use of a coiled optical fiber is desirable because the amount of phase difference shift due to rotation, and so the output signal, depends on the length of the entire optical path through the coil traversed by the two opposing directional electromagnetic waves, and so a large phase difference shift can be obtained in the long optical fiber but in the relatively small volume taken by it in being coiled.
The output current from the photodetector system photodiode in response to the opposite direction traveling electro magnetic waves impinging thereon, after passing through the coiled optical fiber, follows a cosine function. That is, the output current depends on the cosine of the phase difference between these two waves. Since a cosine function is an even function, such an output function gives no indication as to the relative direction of the phase difference shift, and so no indication as to the direction of the rotation about the axis. In addition, the rate of change of a cosine function near zero is very small, and so such an output function provides very low sensitivity for low rotation rates.
Because of these unsatisfactory characteristics, the phase difference between the two electromagnetic waves is usually modulated by placing an optical phase modulator on one side of the coiled optical fiber. As a result, one of the opposite direction propagating waves passes through the modulator on the way into the coil, while the other wave transversing the coil in the opposite direction, passes through the modulator upon exiting the coil. In addition, a phase sensitive demodulator is provided to receive the photodetector output current. Both the optical phase modulator and the phase sensitive demodulator are typically operated by a sinusoidal signal generator at the so-called "proper" frequency, but other waveform types of the same fundamental frequency can be used. Other frequencies can also be used.
This "proper" frequency is selected to be that frequency which results in the modulating of one of the waves 180.degree. out of phase with the modulation of the other. This modulation providing 180.degree. of phase difference between the two waves has the effect of eliminating modulator induced amplitude modulation of the resulting photodetector signal. The value of the "proper" frequency can be determined from the length of the optical fiber and the equivalent refractive index therefor.
The resulting signal output of the phase sensitive demodulator follows a sine function, i.e. the output signal depends on the sine of the phase difference between the two electromagnetic waves impinging on the photodiode, primarily the phase shift due to rotation about the axis of the coil. A sine function is an odd function having its maximum rate of change at zero, and so changes algebraic sign on either side of zero. Hence, the phase sensitive demodulator signal can provide both an indication of which direction a rotation is occurring about the axis of the coil, and can provide the maximum rate of change of signal value as a function of rotation rate near a zero rotation rate, i.e. has its maximum sensitivity near zero phase shifts, so that its output signal is quite sensitive to low rotation rates. This is possible, of course, only if phase shifts due to other sources, that is, errors, are made sufficiently small. In addition, this output signal in these circumstances is very close to being linear at relatively low rotation rates. Such characteristics for the output signal of the phase sensitive demodulator are a substantial improvement over the characteristics of the output current of the photodetector.
Nevertheless, the phase sensitive demodulator output, in following a sine function, results in an output that at rotation rates further from zero, is less and less linear. For rotation rates of an amplitude sufficient to be past one of the peaks of the sine function, the output response value because of being periodic will be ambiguous with respect to just which rotation rate is occurring. Thus, there is a strong desire to operate the gyroscope so that the output signal of the phase sensitive demodulator stays within the linear region near the zero rotation rate value.
This can be accomplished by adding a further phase modulator, or frequency shifter, near the coil in an optical path portion used by the opposite direction traveling electromagnetic waves propagating through the coiled optical fiber to reach the photodetector. This phase modulator, or frequency shifter, is operated in a feedback loop from the photodetector system, and provides sufficient negative feedback such that the phase modulator introduced phase change is just enough to cancel the phase shift difference between the opposite traveling direction electromagnetic waves resulting from a rotation about the axis of the coiled optical fiber. As a result, there will be little phase shift difference occurring at the photodetector except for transient rotation rate changes, and so little phase shift sensed by the phase sensitive demodulator. Thus, the output signal of this phase sensitive demodulator will always be near to, or at, zero. The signal from a generator connected to the phase sensitive demodulator for operating this additional phase modulator, through providing a signal directing the modulator to provide a particular phase shift sufficient to cancel the phase shift due to rotation, will thus contain within it or a related signal the information as to the magnitude and direction of the rotation rate.
Several forms for the output signal from the generator connected to the phase sensitive demodulator in the feedback loop have been suggested for operating this additional optical phase modulator. One common and good choice is to use a serrodyne generator which applies a sawtooth-like signal to the optical phase modulator. A sawtooth or sawtooth-like signal is chosen because it can be shown that a sawtooth signal provides what amounts to a pure frequency translation for the modulated electromagnetic waves, a single-sideband modulator. As a result, light passing through the phase modulator being operated with such a sawtooth signal will leave the modulator with its frequency translated by an amount equal to the frequency of the sawtooth signal. A sawtooth-like signal won't result in pure frequency translation, there instead will be added harmonics generated which can be kept small by providing very nearly a sawtooth waveform and by good design of the modulator.
Since the optical phase modulator so operated will be on one side of the coiled optical fiber, one of the electromagnetic waves will have its frequency translated upon entering the coil while the other will not have its frequency translated until it exits the coil. Hence, one wave traverses the loop having a higher frequency than the other (though both have the same frequency on reaching the photodetector) with the result that, for a fixed modulator (or serrodyne generator) frequency, one will have a phase shift with respect to the other at the photodetector in an amount set by the frequency of the sawtooth and the nature of the fiber of 2.pi..tau..DELTA.f. Here, .DELTA.f is the modulator or generator frequency, and .tau. is the transit time of the light waves through the coil. This phase shift will act to counter the phase shift between the light waves, caused by rotation, because of the negative feedback loop in which the modulator is provided. Thus, the frequency of the sawtooth, or sawtooth-like generator output signal will be an indication of the rotation rate, and the polarity of the sawtooth will indicate the direction of rotation.
However, low rotation rates pose a problem. The lower the rotation rate, the lower the frequency of the sawtooth waveform must be and so the long duration substantially increasing or decreasing amplitude portion of the waveform, or ramp portion, (as opposed to the substantially decreasing or increasing, respectively, "flyback" portion of the waveform) gets very long. This situation results in a waveform which becomes difficult to generate in electronic circuitry. In addition, the gyroscope in these circumstances has a very limited ability in tracking changes in the rotation rate which can occur during the long increasing or decreasing portions of the sawtooth waveform. Such problems will result in slow responses to rotation rate changes, and in a nonlinear or inaccurate scale factor relating the rotation rate to the output signal, a scale factor which is desired to be linear. Thus, there is a strong desire to provide a fiber optic gyroscope which can operate in the linear range of the output signal of the phase sensitive demodulator but which can provide an accurate response to low rotation rates and be able to indicate quickly changes in such rates.